This invention relates to pipeline pig detectors and more particularly to an improved apparatus and method for verification of the position and location of a pig or sphere within a pipeline.
A solid object such as a sphere or scraper, introduced into a pipeline and propelled by the fluid flowing therein, is commonly called a pig. It is sometimes necessary to detect the presence or position of a pig for various reasons. Typically, pig detectors employ a mechanical sensing means, such as a movable trigger, to contact the pig. The trigger is mechanically coupled to a reporting means, such as a switch or valve, and is adapted to actuate this reporting means when the trigger is moved by the pig. Excellent examples of pig detectors are disclosed in U.S. Pat. Nos. 3,145,274 and 4,079,619. Such detectors adequately perform the gross function of reporting the approximate location of a pig in the pipeline.
In the pipeline industry, it is common practice to employ batching pigs (or spheres) for separating two different fluids being successively transmitted through the pipeline. These pigs are first loaded into a specially designed oversized portion of the pipeline to permit ease of loading and controlled launching. To verify that the pig did indeed launch into the flowing pipeline, detectors such as those disclosed in U.S. Pat. Nos. 3,145,274; 3,421,124; and 3,109,410 are mounted on the pipeline a short distance from the oversized launcher and are triggered by the passage of the pig to cause an indication.
When the pig reaches the end of a run, it enters another specially designed oversized portion of the pipeline, commonly called a receiver. A detector, similar to those cited above, is usually mounted on the pipeline just before the oversized receiver to indicate the arrival of the pig into the receiver.
In many instances, the pipeline will be required to operate bi-directionally, such as in tanker or barge loading and unloading, in which case the specially designed oversized ends of the pipeline function as pig launchers as well as pig receivers.
Many installations are now in use where a plurality of pipeline spherical pigs are controlled by retractable restraining pins. The operation of the pin mechanism is usually semi-automatic, being controlled by push-button operation at a location remote from the launching or receiving barrels. For this type of operation, reliable pig passage indication is most important. Often multiple detectors are incorporated when using detectors such as in U.S. Pat. No. 3,421,124, or a plurality of indicators are employed such as in the use of detectors shown in U.S. Pat. No. 3,145,274. In the event of a detector indication failure, which could be caused by a mechanical malfunction, electrical power failure, or even operator error or carelessness in resetting equipment, a pig can become "lost" in the system, and personnel must be dispatched to the launcher or to the receiver to close off the system from the stream, drain and open the unit and physically inspect to determine if the pig or pigs are in the barrel.
The installation of detectors, such as shown in U.S. Pat. No. 3,109,410, on the oversized launching and receiving chambers has been used as a secondary verification in an effort to circumvent the previously stated problems. These installations have not been successful as oscillations of the pig (especially in the case of spherical pigs) in the oversized launcher or receiver, caused by the flowing median, can cause multiple indications and even break the extended indicator trigger.
Present state-of-the-art computer controlled batch separating systems rely on electrical signals from the launching and receiving barrels which are classified as hazardous areas, and electrical wiring is preferred to be installed in rigid metal conduit at these locations. Electrical wiring must be housed in flexible explosion-proof conduits which have extreme bulk and relative stiffness thereby rendering their use impractical with pig detectors. The use of very low voltage/current "intrinsically safe" electrical systems for wiring to the switch contacts in a pig detector would require the flexing of this wiring as the detectors are inserted and retracted thereby eventually causing a falure of the wire. Further, the wire would be exposed and subjected to external damage.
Soft foam pigs are used in some pipeline applications. Prior art detectors are insufficiently sensitive to detect such soft foam pigs.
The method and apparatus of the present invention overcomes the deficiencies of the prior art.